# Homework Help: Trajectory of an electron in cathode ray tubes

1. Jul 16, 2011

### mattg443

Why is it that an electron travelling under the influence of a magnetic field (i.e electron going from left to right and B field going into the page) will travel in the arc of a circle

yet when an electric field alone is applied (say up page and the direction of motion of the electron is the same) the electron will travel in a parabolic path? when in both cases, a force is exerted on the electron is downwards?

2. Jul 17, 2011

### ehild

The force the magnetic field exerts on a moving charged particle is perpendicular to both the magnetic field and the velocity of the particle. The acceleration is normal to the velocity: only the direction of the velocity changes, its magnitude does not. This is circular motion.

3. Jul 17, 2011

### elegysix

The force due to the magnetic field is not constantly downwards. It may initially be downwards, but as it changes the direction of the velocity, the force will also change direction; even though the magnetic field remains the same.

the force from a magnetic field is given by
F=qVXB (resulting in circular motion)

and for the electric field alone,
F=qE.

Bold letters are vectors. hope this helps!